Electrostatic pinning of dielectric film

ABSTRACT

Apparatus for electrostatically pinning a moving dielectric web to a roll which comprises a first electrode and a second, insulated electrode independent of the roll, and means for establishing a potential difference therebetween.

its States atent awkins [4 1 A 1'. H 1972 54] ELECTROSTATIC PIN NING 0F[56] References Cited DIELECTRIC FILM UNITED STATES PATENTS [72] InventHawkins Ohm 3,427,686 2/1969 Busby ..18/15 M [73] Assignee: E. I. duPont de Nemours and Company, 3,520,959 7/1970 Busby ..l8/l5 R XWilmington, Del. P Ex J S o h is rzmary ammerpencer ver 0 er [22] Filed:1970 Assistant Examiner-Norman E. Lehrer [21] AppL NOJ 21,695Attorney-Donald W. Huntley Related 0.5. Application Data 1 ABSTRACT [63]Continuatiommpan of 823 848 May 12 Apparatus for electrostaticallypinning a moving dielectric 1969 abandoned web to a roll which comprisesa first electrode and a second, insulated electrode independent of theroll, and means for [52] U 8 Cl 425/109 264/22 264/24establishingapotential difference therebetween.

425/174, 425/151, 425/7 25/162 9 Claims, 6 Drawing Figures [51] Int. Cl..B29d 7/22 [58] FieldoiSearch ..l8/l5,l5R,15F,15S;

PATENTEDAPR 1 1 I972 SHEET 1 BF 2 FIG"? m T N E V m "WILLIAM E. HAWKINSBYQ QW ATT may PATENTEDAPR 11 I972 3.655.307

SHEET 2 [1F 2 INVENTOR WILLIAI E. HAWKINS ATTORNEY ELECTROSTATIC PINNINGOF DIELECTRIC FILM CROSS-REFERENCE TO RELATED APPLICATION This is acontinuation-in-part of copending application Ser. No. 823,848, filedMay 12, 1969, now abandoned.

BACKGROUND OF THE INVENTION In the preparation and treatment ofdielectric films, it is often necessary to apply a force to the film tobring or hold it in contact with rolls or belts. For example, films areregularly subjected to coating and stretching treatments where firmcontact with rollers is essential to the operation, but where theapplication of conventional mechanical forces would be undesirable Oneof the most widely used methods of pinning film involves imparting anelectrostatic charge to the film through an electric field between thewire or point electrode and the grounded surface on which the film iscarried. However, for certain high-speed operations, even electrostaticpinning is less than completely satisfactory.

Certain unique requirements have been encountered, for example in thecasting of molten, crystallizable thermoplastic web, where it isnecessary to quickly cool the molten web to a temperature below theglass transition temperature to minimize crystallization. The extrudedweb is generally cooled by casting the molten thermoplastic materialonto a moving chilled surface, using electrostatic pinning to bring theweb into intimate contact with the chilled surface. Previous attempts toincrease the speed of this procedure for more efficient and economicaloperation have resulted in poor gauge uniformity and regularly recurringhaze patterns known in the art as venetian blind haze.

The higher speeds are believed to result in the entrapment of airbetween the drum and the web, which hinders quenching by diminishingheat transfer between the drum and the web. Attempts have previouslybeen made to increase the electrostatic force generated by the wire orprobes by increasing the voltage. These attempts, however, have for themost part been ineffective, since increased voltage generally causes acatastrophic electrical breakdown between the electrode and the web longbefore any substantial increase in the pinning force is effected. Thesparking between the electrode and the surface of the web or other partsof the apparatus interrupts the electrical field which contributes tothe pinning force. In addition, the sparking can'cause pinholes in afreshly cast soft web, which holes are greatly enlarged by anorientation of the film.

Consequently, the pinning force available through electrostatic pinningmeans has heretofore been inadequate for some high-speed applications.

SUMMARY OF THE INVENTION The instant invention provides an apparatus forelectrostatic pinning which can produce a substantially higher and moreuniform pinning force than has heretofore been available.

Specifically, the instant invention provides an apparatus for pinning adielectric film to an electrically grounded moving surface whichcomprises a first electrode and a second, grounded electrodeelectrically insulated from the first electrode, a high voltage currentsource and means connecting the current source to the first electrode toestablish an electrical potential difference between the first andsecond electrodes, the first electrode being in spaced relationship tothe moving surface, the distance between the first and second electrodesbeing less than the distance between the first electrode and the movingsurface. Preferably the apparatus further comprises means forsubstantially surrounding at least the first electrode with a gas.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 2, and 3 are schematicrepresentations of crosssectional views of specific embodiments of theapparatus of the instant invention, shown in conjunction with extrusionand quenching apparatus; and

FIG. 4 is a view in perspective of an apparatus of FIG. 3; FIGS. 5 and 6are cross-sectional views of additional specific embodiments of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The first electrode used in theinstant invention can be made from any metallic conductor havingadequate strength and dimensional stability to withstand the stressesencountered in the operation of the instant process. Such materials caninclude, for example, tempered steel, tungsten, Inconel nickel-ironalloy, Monel nickel alloy, copper, brass, and stainless steel. Theparticular configuration of the first electrode can be similar to thatof electrodes generally used in the art of electrostatic pinning, suchas those described in Owens et al., US. Pat. No. 3,223,757, herebyincorporated by reference. .For full sheet electrostatic pinning, a thinwire, substantially circular in cross-section, is especially preferreddue to the exceptionally high rate of ion formation believed to resultfrom the minimum surface area. However, in those embodiments of theinvention using a knife edge or a plurality of pointed electrodes asdescribed in the Owens et al. specification, the functional edge of theelectrode should be directed toward the second electrode of the instantapparatus instead of the quenching surface as required in Owens et al.

The primary electric field for the generation of pinning ions isestablished between a first electrode, described above, and a second,grounded electrode. Accordingly, an important feature of the instantinvention is that the distance between the first and second electrodesbe less than the distance between the first electrode and the groundedmoving surface. Outside of this basic requirement, the positioning ofthe second electrode can vary widely. However, since the point of ionformation is believed to be at or near the first electrode, the straightlinear paths between the first electrode and the grounded moving surfaceshould be substantially unimpeded, particularly prior to the point ofinitial or strongest contact of the dielectric and the grounded movingsurface. Thus, while a rod-like or relatively small second electrode canbe beneficially placed between the first electrode and the groundedmoving surface for the acceleration of ions toward that surface, it isgenerally preferred that the second electrode be placed farther from thedielectric film than the first electrode so as to minimize interferencewith the deposition of ions onto the film. It is also preferred that thedistance between the first and second electrodes be less than about 1inch, for optimum ion formation.

In addition to the higher pinning force obtainable through the use ofthe two electrodes in accordance with the instant invention, anotheradvantage can be realized through a specific position of the secondelectrode. It has been found that the charging of a freshly extrudedthermoplastic web along the span extending from an extrusion orifice tothe touchdown point onto a quenching surface irregularly increases theattraction of the web to the quenching surface so as to result inincreased variation of the touchdown point along the width of the film.This variation in the touchdown point can result in the intermittententrapment of small air bubbles, an effect often encountered at highquenching speeds. Accordingly, the second electrode is preferablypositioned so as to prevent charging of the web substantially before thetouchdown point onto the drum. Such positioning is illustrated in FIGS.5 and 6 wherein electrodes 14 and 18, together with insulating layers 17and 19 are positioned so as to intersect a substantial percentage, andpreferably at least about 50 percent, of the straight linear pathsbetween wire electrode 13 and the freshly extruded web 10. It may benoted that this positioning does not interfere with the straight linearpaths between the first electrode 13 and the quenching surface 12, theextreme linear paths being defined by tangents to the quenching surfacefrom the wire.

A gap should be maintained between the second electrode and the freshlyextruded web sufficient to prevent the web contacting the secondelectrode due to the aerodynamic effects of the moving web. A set gap ofat least about /5 inch is generally sufficient to prevent contact.

The second electrode can be prepared from the same materials suggestedabove for use in the first electrode. In addition, the second electrodeis insulated from the first electrode by a dielectric material. It isfound that the dielectric insulation of the second electrode preventsthe discharge of gaseous ions on the second electrode and enables theapplication of exceptionally high voltages to the first electrodewithout a breakdown in the gap between the electrodes, thus preventinginterruption of the pinning force. In addition, ions attracted to thegrounded second electrode retain their positive or negative charge,thereby repelling like ions toward the web to effect the desiredpinning.

The degree to which the second electrode is covered by the dielectricmaterial will vary with the particular configuration of the secondelectrode. For purposes of the instant invention, the second electrodeis considered to be insulated when all of the straight linear pathsbetween the first and second electrodes are intersected by dielectric.

Dielectric material which can be used to insulate the second electrodeinclude natural and synthetic rubbers as well as resinous materials suchas polyimides, fluorocarbon resins, urea formaldehyde resins, phenolformaldehyde resins, nylons and cast epoxy resins. It has been foundthat Teflon fluorocarbon resins are particularly well suited for thisapplication due to their excellent electrical insulating properties andstability at elevated temperatures.

The particular physical configuration of the second electrode is notcritical to the instant process, as long as the second electrode doesnot surround the first electrode to the extent of intersecting asubstantial percentage of the straight linear paths between the firstelectrode and the moving surface. Accordingly, the second electrode, ina basic form, can be a metal rod, as illustrated in FIG. 1. In thatfigure, a thermoplastic web is extruded from hopper 11 onto a movingquench drum 12. A first electrode 13 and a second electrode 14 arepositioned approximately above the touchdown point 15 of the web ontothe drum. The first electrode is connected to high voltage source 16 andthe second electrode is insulated by sheath 17.

The second electrode, in a preferred embodiment of the invention, isarcuate in cross section and is positioned farther from the web than thefirst electrode, with the first electrode positioned within the arcdefined by the second electrode. This particular embodiment facilitatesthe direction of the ions formed by the electric field toward thethermoplastic web. One such embodiment of this type is illustrated inFIG. 2 wherein arcuate second electrode 18 is positioned adjacent firstelectrode 13 and is insulated therefrom by a dielectric insulatingsheath 19. The arcuate configuration of the second electrode permits theuse of an insulating barrier on only one side of the second electrode,while intersecting all of the straight linear paths between the twoelectrodes.

Both the second electrode and the moving surface in the instantinvention are generally grounded to provide the requisite potentialdifference between the first and second electrode and the attraction ofthe formed ions by the drum. The grounding can be direct or throughnominal resistance, such as the framework of the mechanism involved.

The high voltage source applied to the first electrode can have avoltage of about from 2 to kilovolts and an amperage of about from 1 to3,000 microamperes, for electrodes generally used for pinningoperations. In general, a unidirectional current, and especially aunidirectional positive current are preferred.

In a preferred embodiment of the instant invention, a gas stream isdirected so as to substantially surround at least the first electrode.The gas is believed to contribute to the pinning force by increasing thecurrent generated on the wire without causing sparking. Accordingly,gases which can be used in this embodiment are selected from those inwhich a Wire Current Before Breakdown of at least aboutmicroamperes/inch of wire can be generated, as measured by the CurrentGeneration Test. The requirements of the Current Generation Test, aswell as specific gases in which the required Wire Current BeforeBreakdown can be generated are set forth in full detail in copending,coassigned US. Pat. application Ser. No. 21,696, (F-l988-R) filedsimultaneously herewith, which is hereby incorporated by reference.

It is preferred that the difference between the Voltage at ThresholdCurrent and the Voltage at Spark Breakdown of the gas be at least 2.0kilovolts, This minimizes the need for close control of the voltageapplied to the pinning apparatus to prevent sparking.

Preferred gases in which a Wire Current Before Breakdown of at least 100microamperes per inch of wire can be generated include nitrogen, helium,air having a moisture content of less than about 5 percent oxygen, Freonl 14 dichlorotetrafluoroethane, Freon l2 dichlorodifiuoromethane, airhaving a moisture content of less than 5 percent and substantiallysaturated with carbon tetrachloride at room temperature, andtetrachloroethane.

In those embodiments where the second electrode is of a semicircular orarcuate configuration, the gas is best directed so as to surround thefirst electrode as well as pass over the insulated surface of the secondelectrode facing the first electrode. An apparatus illustrating thislattermost embodiment is shown in FIG. 3, wherein a gaseous stream 30supplied from gas source 31 is passed through second electrode 32 anddielectric insulator 33 to escape along the surface of the insulatinglayer and over first electrode 13 onto the thermoplastic web.

The gas should be supplied at a minimal pressure necessary to maintainan atmosphere around the electrodes, since extremely high gas pressurecan cause distortion of the web in some applications.

The apparatus of the instant invention is applicable to the pinning ofany dielectric film. Preferred films include those of organicthermoplastic polymer including, for example, polyesters such aspolyethylene terephthalate, polyolefins such as polyethylene andpolypropylene, vinyl acetate polymers and copolymers, vinylidenechloride polymers and copolymers, polyamides, cellulosic esters andethers, styrene polymers and copolymers, rubber hydrochlorides andpolycarbonates. The apparatus is particularly applicable to thequenching of crystalline polymers and especially polyethyleneterephthalate, since the drawing and resulting optical properties ofthese films, when produced at high speeds, are greatly enhanced.

The instant apparatus is additionally applicable to the handling ofother dielectric films, for example, in coating or printing operationsfor papers, cellophane, and thermosetting resins such as Kapton"polyimide resin.

The instant invention provides a higher and substantially more uniformpinning force than has heretofore been available through the use ofknown methods of electrostatic pinning. The reasons for the improvedpinning are not fully understood, but are believed to be primarily afunction of the independence of the pinning apparatus from the groundedroll to which the dielectric film is pinned and the insulation of thesecond electrode. The instant apparatus is found to provide increasedpinning force even when positioned substantially further away from thedielectric film than has been possible using apparatus arrangementswherein the primary electrical field is established between an electrodeand a grounded roll. Further, the apparatus is relatively independent ofexternal mechanical vibration, resulting in a more uniform generationand deposition of gaseous ions onto the thermoplastic film.

The higher voltages which can be used in the instant invention and thehigher pinning forces which can be obtained permit the use ofcorrespondingly higher operating speeds, as illustrated by the followingexamples.

Polyethylene terephthalate is extruded and pinned to a quench drumsubstantially as described in Example I of Owens et al., U.S. Pat.3,223,757, to establish a control standard. The film is thereafterbiaxially oriented to a thickness of about 0.75 mil. The maximum rate ofproduction for films of consistently high quality is determined anddesignated as R.

The procedure is repeated, using a pinning apparatus of the typeillustrated in FIG. 3 of the instant specification. The first electrodeis a steel wire having a diameter of 0.006 inch. The second, arcuateelectrode is steel and is positioned about 0.6 inch away from the firstelectrode. The second electrode has a mil insulating layer ofpolytetrafluoroethylene resin on the concave surface. The firstelectrode is about 0.75 inch away from the drum. A positiveunidirectional current of about 2 milliamperes having a voltage of about18 kilovolts is applied to the first electrode. Oxygen is supplied tothe apparatus through the aperture formed in the second electrode andits insulating layer at a minimal pressure necessary to maintain acontinuous stream around the first electrode.

The maximum production rate using the pinning apparatus of the instantinvention is determined, and found to be 1.60R.

EXAMPLES 21 I In Examples 2 to l 1, polyethylene terephthalate is meltextruded at a constant rate onto a cooled quench drum. The quench drumhas a diameter of 6 feet and the extruded sheet is 16% inches wide. Thethickness of the film on the quench drum varies with the speed of thedrum.

In Example 2, a single 0.008 inch diameter stainless steel wireelectrode is situated inch above the quench drum sur face at a positionwhich gives the best possible pinning effect. A positive unidirectionalvoltage is applied to the wire at the highest voltage possible withoutsparking, and the speed of the drum is increased to the greatest ratepossible without the appearance of pinner bubbles between the surfacesof the drum and the extruded web. The results of this control standardare summarized below.

Film Maximum Current thickness drum speed Voltage (pa/in.

Example G as (mils) (ft/min.) (kv.) wire) .2 Room air. 7. 4 80 9. 2 30In Examples 3-5, the procedure of Example 2 is repeated, except that anelectrode apparatus of the type illustrated in FIG. 3 is used instead ofthe bare wire electrode of Example 2. The electrode apparatus ispositioned at the same point as the bare electrode, and the same voltageis applied to the wire. In Examples 4 and 5, oxygen and nitrogen aresupplied to the apparatus to substantially surround the pinning wire.These gases effect a moderate increase in the amount of ions formedwhich causes an intermittent force increase across the width of thefilm. This causes variations in the touchdown point which actuallydecreases the maximum operating speed without pinner bubbles.

Film Maximum Current thickness drum speed Voltage a /in. Example Gas(mils) (ft./min.) (kv.) wire) 3 Room air- 6.0 110 9.2 37 4.. Oxygen...6.0 105 9.2 44 5 Nitrogen" 6.0 105 0. 2 80 touchdown point and efi'ectan increase in the maximum speed without pinner bubbles.

Film Maximum Current thickness drum speed Voltage (um/in.

Example Gas (mils) (ft./min.) (kv.) wire) 6. Room an 5.0 10. 5 89 7....Oxygen... 5.0 10.8 04 8. Nitrogen. 5.0 135 11.3 81

In Examples 9-1 1, the procedure of Examples 6-8 is repeated, exceptthat the position of the pinning apparatus is adjusted to the bestpossible, as opposed to placing the apparatus at the point of bestperformance of the bare wire.

1. An apparatus for pinning a dielectric film extruded from a hopperonto an electrically grounded moving surface which comprises a firstelectrode and a second, grounded electrode spaced from said hopper andelectrically insulated from the first electrode by a dielectric coatingthereon which intersects substantially all of the straight linear pathsbetween the first and second electrodes, a high voltage current sourceand means connecting the current source to the first electrode toestablish an electric potential difference between the first and secondelectrodes, the first electrode being in spaced relationship to themoving surface, the distance between the first and second electrodesbeing less than the distance between the first electrode and the movingsurface.

2. An apparatus of claim 1 wherein the high voltage source is aunidirectional current source.

3. An apparatus of claim 2 wherein is positive.

4. An apparatus of claim 1 wherein the distance between the firstelectrode and the moving surface is less than the distance between thesecond electrode and the moving surface.

5. An apparatus of claim 1 further comprising means for substantiallysurrounding at least the first electrode with gas.

6. An apparatus of claim 1 wherein the electrically grounded movingsurface is a quenching surface and the film is thermoplastic,melt-extruded onto the quenching surface from a die orifice.

7. An apparatus of claim 6 wherein the second electrode is so positionedas to intersect at least 50 percent of the straight linear paths betweenthe first electrode and that portion of the melt-extruded film extendingfrom the die orifice to the point of touchdown of the film onto thequenching surface.

8. An apparatus of claim 1 wherein the second electrode is arcuate inconfiguration and is positioned farther from the moving surface than thefirst electrode, the concave surface of the second electrode facing themoving surface, the second electrode being insulated from the firstelectrode by a layer of dielectric resin on at least the concave surfacethereof and wherein the first electrode is positioned within the arcdefined by the second electrode.

9. An apparatus of claim 7 wherein the second electrode and the layer ofdielectric resin have at least one aperture formed therethrough, theapparatus further comprising means for passing a gaseous stream throughthe aperture and toward the first electrode and the film.

the high voltage source

1. An apparatus for pinning a dielectric film extruded from a hopperonto an electrically grounded moving surface which comprises a firstelectrode and a second, grounded electrode spaced from said hopper andelectrically insulated from the first electrode by a dielectric coatingthereon which intersects substantially all of the straight linear pathsbetween the first and second electrodes, a high voltage current sourceand means connecting the current source to the first electrode toestablish an electric potential difference between the first and secondelectrodes, the first electrode being in spaced relationship to themoving surface, the distance between the first and second electrodesbeing less than the distance between the first electrode and the movingsurface.
 2. An apparatus of claim 1 wherein the high voltage source is aunidirectional current source.
 3. An apparatus of claim 2 wherein thehigh voltage source is positive.
 4. An apparatus of claim 1 wherein thedistance between the first electrode and the moving surface is less thanthe distance between the second electrode and the moving surface.
 5. Anapparatus of claim 1 further comprising means for substantiallysurrounding at least the first electrode with gas.
 6. An apparatus ofclaim 1 wherein the electrically grounded moving surface is a quenchingsurface and the film is thermoplastic, melt-extruded onto the quenchingsurface from a die orifice.
 7. An apparatus of claim 6 wherein thesecond electrode is so positioned as to intersect at least 50 percent ofthe straight linear paths between the first electrode and that portionof the melt-extruded film extending from the die orifice to the point oftouchdown of the film onto the quenching surface.
 8. An apparatus ofclaim 1 wherein the second electrode is arcuate in configuration and ispositioned farther from the moving surface than the first electrode, theconcave surface of the second electrode facing the moving surface, thesecond electrode being insulated from the first electrode by a layer ofdielectric resin on at least the concave surface thereof and wherein thefirst electrode is positioned within the arc defined by the secondelectrode.
 9. An apparatus of claim 7 wherein the second electrode andthe layer of dielectric resin have at least one aperture formedtherethrough, the apparatus further comprising means for passing agaseous stream through the aperture and toward the first electrode andthe film.